"TonyL" wrote in message


I want to drive low-z (32 ohm) headphones using op amps.

For various reasons I need:

1) Quad amps to be used, there's a requirement for other
op-amps in the units.

That one is probably a non-starter, for reasons I'll detail below.

2) Single rail, around 12-15 V, I guess most modern op
amps will do that easily.

Yes.

3) Not too current hungry, I want to power a number of
these units from one simple 12VDC transformer supply.

The application defines the current drain.

4) Lo-fi will do so long as distortion is not too gross.
Intercom quality is OK.

No need to compromise sound quality.

5) Reasonable cost.

I built a prototype using a LM324 that happened to be
handy. Result was gross crossover distortion when the
output had to supply current to the low-z earphones.

Read the spec sheet and do the math. What sort of voltage will you need to
drive headphones? How much peak current does this net out to be?

I know these amps can be biased into class A operation but
I want reasonable power economy. Would be nice to just
drop a pin compatible device into my prototype, say a LM
387 ? Except the spec says
600ohms load.

Advice/comments please ?

Quit pulling numbers out of your memory from days long ago, and do what
professional engineers do when they have a task like this: Go to several op
amp manufacturer sites like TI, National, Maxim, NJM and Philips, and see
what they recommend for the purpose.

There are even such things as op amps that are designed to drive low
impedance loads and work from single-ended supplies.

The odds that they will be quads is about zero, since headphone drivers are
typically used in pairs.

"Arny Krueger" wrote in message
...
"TonyL" wrote in message


I want to drive low-z (32 ohm) headphones using op amps.

snip

There are even such things as op amps that are designed to drive low
impedance loads and work from single-ended supplies.

Indeed there are. Since 32ohm is now the standard for headphones there are
plenty of heaphone amps intended to drive 32 ohm h/phones on the market.
Mostly they are designed to work from single-ended supplies of around 3-12V

The odds that they will be quads is about zero, since headphone drivers
are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony should
accept that he will need a separate h/phone amp chip.

David.

David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the crossover
distortion. It drives low-z headphones fine. So, I'll try plugging in a
TL074 and see what happens before looking to a purpose designed hadphone
driver.

In article , TonyL
scribeth thus
David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the crossover
distortion. It drives low-z headphones fine. So, I'll try plugging in a
TL074 and see what happens before looking to a purpose designed hadphone
driver.



Would one of those SSM -whatever -there -called series line drivers do
this quite well?..
--
Tony Sayer

tony sayer wrote:
In article , TonyL
scribeth thus
David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the
crossover distortion. It drives low-z headphones fine. So, I'll try
plugging in a TL074 and see what happens before looking to a purpose
designed hadphone driver.



Would one of those SSM -whatever -there -called series line drivers do
this quite well?..


Don't know. But if I can't use the spare section on the existing quad op-amp
to drive the headphone then I may as well go for a purpose designed heaphone
amp as suggested by others in here, or add a bipolar complementary pair.

TonyL wrote:

Don't know. But if I can't use the spare section on the existing quad op-amp
to drive the headphone then I may as well go for a purpose designed heaphone
amp as suggested by others in here, or add a bipolar complementary pair.

Yes, buffer the op-amp with a complementary pair. Don't forget to bias them on
either and set the current with emitter resistors. You can even take the
feedback from the junction of the emitter resistors but be advised to include a
load 'isolating' resistor of say 47 ohms.

Graham

Eeyore wrote:
TonyL wrote:

Don't know. But if I can't use the spare section on the existing
quad op-amp to drive the headphone then I may as well go for a
purpose designed heaphone amp as suggested by others in here, or add
a bipolar complementary pair.

Yes, buffer the op-amp with a complementary pair. Don't forget to
bias them on either and set the current with emitter resistors. You
can even take the feedback from the junction of the emitter resistors
but be advised to include a load 'isolating' resistor of say 47 ohms.


Thanks,

Probably the simplest solution for this application, a few pence worth of
discrete devices. Your tips regarding class AB biasing+feedback+load
isolator resistors duly noted.

TonyL wrote:

David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the crossover
distortion.

A consequence of its internal design.

Graham

On Tue, 13 Jan 2009 08:22:26 +0000, Eeyore
wrote:



TonyL wrote:

David Looser wrote:

The odds that they will be quads is about zero, since headphone
drivers are typically used in pairs.

Agreed. I know of no quad op-amps that will do the job. I think Tony
should accept that he will need a separate h/phone amp chip.

All comments noted. Thanks.

The *only* problem with the LM324 in this application is the crossover
distortion.

A consequence of its internal design.

Graham

It has a poorly biassed output stage - virtually pure class B.

d

Don Pearce wrote in message news:496c541a.346449453@localhost...
On Tue, 13 Jan 2009 08:22:26 +0000, Eeyore
wrote:


TonyL wrote:


The *only* problem with the LM324 in this application is the crossover
distortion.

A consequence of its internal design.

Graham

It has a poorly biassed output stage - virtually pure class B.


Whether the design is "poor" or not depends on what application you have in
mind. THe LM324 was one of the first op-amp designs to work from a single 5V
rail, making it a convenient way of adding analogue functions to a logic
board. The input common-mode range includes the -ve supply rail making it a
good choice for comparators etc., and the output can drive TTL directly.

And the LM324 *can* work quite well as an audio amp (though possibly not a
headphone driver) as long as a suitable resistor is connected between the
output pin and the -ve supply rail.

I entirely agree that there are far better audio op-amps around these days,
but the LM324 still has it's uses, and it's dirt cheap!

David.